Thermal mass transfer donor element

ABSTRACT

A thermal mass transfer donor element is provided that includes a thermal transfer layer and a light-to-heat conversion layer, wherein the light-to-heat conversion layer has at least two regions exhibiting different absorption coefficients. The thermal transfer donor elements provided can improve imaging performance by increasing transfer sensitivity and decreasing imaging defects.

This application is a divisional of U.S. patent application Ser. No.09/474,002, filed Dec. 28, 1999, now U.S. Pat. No. 6,228,555.

This invention relates to thermal mass transfer donor elements fortransferring materials to a receptor.

BACKGROUND

The thermal transfer of layers from a thermal transfer element to areceptor has been suggested for the preparation of a variety ofproducts. Such products include, for example, color filters, spacers,black matrix layers, polarizers, printed circuit boards, displays (forexample, liquid crystal and emissive displays), polarizers, z-axisconductors, and other items that can be formed by thermal transferincluding, for example, those described in U.S. Pat. Nos. 5,156,938;5,171,650; 5,244,770; 5,256,506; 5,387,496; 5,501,938; 5,521,035;5,593,808; 5,605,780; 5,612,165; 5,622,795; 5,685,939; 5,691,114;5,693,446; and 5,710,097 and International Publication Nos. WO 98/03346and WO 97/15173, incorporated herein by reference.

For many of these products, resolution and edge sharpness are importantfactors in the manufacture of the product. Another factor is the size ofthe transferred portion of the thermal transfer element for a givenamount of thermal energy. As an example, when lines or other shapes aretransferred, the linewidth or diameter of the shape depends on the sizeof the resistive element or light beam used to pattern the thermaltransfer element. The linewidth or diameter also depends on the abilityof the thermal transfer element to transfer energy. Near the edges ofthe resistive element or light beam, the energy provided to the thermaltransfer element may be reduced. Thermal transfer elements with betterthermal conduction, less thermal loss, more sensitive transfer coatings,and/or better light-to-heat conversion typically produce largerlinewidths or diameters. Thus, the linewidth or diameter can be areflection of the efficiency of the thermal transfer element inperforming the thermal transfer function.

SUMMARY OF THE INVENTION

One manner in which thermal transfer properties can be improved is byimprovements in the formulation of the transfer layer material. Forexample, co-assigned U.S. patent application Ser. No. 09/392,386, nowU.S. Pat. No. 6,228,543, discloses including a plasticizer in thetransfer layer to improve transfer properties. Other ways to improvetransfer fidelity during laser induced thermal transfer includeincreasing the laser power and/or fluence incident on the donor media.However, increasing laser power or fluence can lead to imaging defects,presumably caused in part by overheating of one or more layers in thedonor media.

The present invention recognizes problems associated with trying toimprove the sensitivity of thermal transfer and offers new approaches.The present invention provides improved constructions for thermal masstransfer donor elements, specifically providing new light-to-heatconversion layer (LTHC) constructions. The constructions and methods ofthe present invention can be used to provide thermal transfer donorelements that exhibit, for example, higher transfer sensitivity, fewerimaging defects (e.g., those defects related to donor elementover-heating), and the like.

In one embodiment, the present invention provides a thermal masstransfer donor element that includes a thermal transfer layer and alight-to-heat conversion layer, wherein the light-to-heat conversionlayer has at least two regions exhibiting different absorptioncoefficients. For example, the absorption coefficient can vary throughthe thickness of the light-to-heat conversion layer.

In another embodiment, the present invention provides a thermal masstransfer donor element that includes a thermal transfer layer and anon-homogeneous light-to-heat conversion layer, where the donor elementis capable of being used for imagewise thermal mass transfer of materialfrom the transfer layer to a receptor when the donor element is exposedto imaging radiation that can be absorbed and converted into heat by thenon-homogeneous light-to-heat conversion layer. The non-homogeneouslight-to-heat conversion layer is provided so that, for a set of imagingconditions, improved imaging properties can be attained (such as a lowermaximum temperature, improved imaging sensitivity, increased imagingfidelity, and decreased imaging defect formation) compared to anotherwise nearly identical donor element that includes a homogeneouslight-to-heat conversion layer that has a thickness and optical densitythat are the about same as for the non-homogeneous light-to-heatconversion layer.

In still another embodiment, the present invention provides a method forimproving the imaging properties of thermal mass transfer donor media byproviding a substrate and a thermal transfer layer, and then forming alight-to-heat conversion layer between the substrate and the thermaltransfer layer, the light-to-heat conversion layer having at least tworegions exhibiting different absorption coefficients.

In yet another embodiment, the present invention provides a method ofthermal mass transfer including the steps of providing a donor elementthat includes providing a donor element that has a thermal transferlayer and a light-to-heat conversion layer, the light-to-heat conversionlayer having at least two regions exhibiting different absorptioncoefficients; placing the thermal transfer layer of the donor elementadjacent to a receptor substrate; and thermally transferring portions ofthe thermal transfer layer from the donor element to the receptorsubstrate by selectively irradiating the donor element.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIGS. 1(a)-(d) show various thermal mass transfer donor elementconstructions;

FIG. 2(a) is a plot showing absorption coefficient profiles as afunction of depth for various light-to-heat conversion layers;

FIG. 2(b) is a plot showing relative power absorbed per unit volume as afunction of depth during imaging of the light-to-heat conversion layersdepicted in FIG. 2(a);

FIG. 2(c) is a plot showing relative temperature profiles as a functionof depth during imaging of the light-to-heat conversion layers depictedin FIG. 2(a);

FIG. 3(a) is a plot comparing line width versus laser dose for linestransferred from a donor element that includes a non-homogeneouslight-to-heat conversion layer and a donor element that includes ahomogeneous light-to-heat conversion layer; and

FIG. 3(b) is a plot comparing edge roughness versus laser dose for linestransferred from a donor element that includes a non-homogeneouslight-to-heat conversion layer and a donor element that includes ahomogeneous light-to-heat conversion layer.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION

The present invention is believed to be applicable to thermal masstransfer of materials from a donor element to a receptor. In particular,the present invention is directed to thermal mass transfer donorelements and methods of thermal mass transfer using donor elements thatinclude an optional substrate, a light-to-heat conversion layer (LTHClayer), and a thermal transfer layer. The LTHC layer can be constructedaccording to the present invention to have a non-homogeneousdistribution of absorber material (e.g., an absorber distribution thatvaries with thickness of the LTHC layer). Using a non-homogeneous LTHClayer can result in a lower maximum temperature attained in the LTHClayer and/or improved imaging properties (e.g., improved transfersensitivity, decreased imaging defect formation, etc.) for a set ofimaging conditions, for example as compared to a similar donor elementthat includes a homogeneous LTHC layer having a thickness and opticaldensity about the same as for a non-homogeneous LTHC layer of thepresent invention. Fidelity refers to the correspondence between anintended transfer pattern and the actual pattern transferred, and can beapproximated by comparing the dimensions of the transferred pattern withthe intended dimensions, and/or by measuring the roughness of the edgesof the transferred pattern, and/or by measuring the area covered by thetransferred pattern and/or by measuring the surface topography of thetransferred pattern.

Using the donor constructions and methods of the present invention canmake it possible to manage temperatures and temperature distributionsattained during imaging of thermal mass transfer donor media, as well asto control thermal transport between and within the layers of donorelements during imaging.

FIGS. 1(a)-(d) show examples of thermal mass transfer donor elementconstructions. While each of the donor constructions shown includes asubstrate, the substrate is an optional component, and if included mayor may not be removed before imaging. Donor element 100 includes a donorsubstrate 110, a LTHC layer 112, a thermal transfer layer 114, and aninterlayer 116 disposed between the LTHC layer and the thermal transferlayer. Donor element 102 includes a donor substrate 110, a LTHC layer112, and a thermal transfer layer 114. Donor element 104 includes adonor substrate 110, a LTHC layer 112, a thermal transfer layer 114, aninterlayer 116 disposed between the LTHC layer and the thermal transferlayer, and an underlayer 118 disposed between the donor substrate andthe LTHC layer. Donor element 106 includes a donor substrate 110, a LTHClayer 112, a thermal transfer layer 114, and an underlayer 118 disposedbetween the donor substrate and the LTHC layer. Each of the includedcomponents, the optional donor substrate 110, the optional underlayer118, the LTHC layer 112, the optional interlayer 116, and the thermaltransfer layer 114, are described in more detail in the discussion thatfollows.

Materials can be transferred from the transfer layer of a thermal masstransfer donor element (such as those shown in FIGS. 1(a)-(d)) to areceptor substrate by placing the transfer layer of the donor elementadjacent to the receptor and irradiating the donor element with imagingradiation that can be absorbed by the LTHC layer and converted intoheat. The donor can be exposed to imaging radiation through the donorsubstrate (or directly onto the LTHC layer if no donor substrate isused), or through the receptor, or both. The radiation can include oneor more wavelengths, including visible light, infrared radiation, orultraviolet radiation, for example from a laser, lamp, or other suchradiation source. Material from the thermal transfer layer can beselectively transferred to a receptor in this manner to imagewise formpatterns of the transferred material on the receptor. In many instances,thermal transfer using light from, for example, a lamp or laser, isadvantageous because of the accuracy and precision that can often beachieved. The size and shape of the transferred pattern (e.g., a line,circle, square, or other shape) can be controlled by, for example,selecting the size of the light beam, the exposure pattern of the lightbeam, the duration of directed beam contact with the thermal masstransfer element, and/or the materials of the thermal mass transferelement. The transferred pattern can also be controlled by irradiatingthe donor element through a mask.

In addition, and as taught by the present invention, the shape of thetransferred pattern and its fidelity to an intended pattern can becontrolled by donor construction design, for example through thedistribution and/or orientation of absorber material in one or morelayers of the donor element, specifically through the distributionand/or orientation of absorber material within the light-to-heatconversion layer, and through relative thermal conductivity values anddirectionalities of the donor element layers.

The mode of thermal mass transfer can vary depending on the type ofirradiation, the type of materials in the transfer layer, etc., andgenerally occurs via one or more mechanisms, one or more of which may beemphasized or de-emphasized during transfer depending on imagingconditions, donor constructions, and so forth. One mechanism of thermaltransfer includes thermal melt-stick transfer whereby localized heatingat the interface between the thermal transfer layer and the rest of thedonor element can lower the adhesion of the thermal transfer layer tothe donor in selected locations. Selected portions of the thermaltransfer layer can adhere to the receptor more strongly than to thedonor so that when the donor element is removed, the selected portionsof the transfer layer remain on the receptor. Another mechanism ofthermal transfer includes ablative transfer whereby localized heatingcan be used to ablate portions of the transfer layer off of the donorelement, thereby directing ablated material toward the receptor. Yetanother mechanism of thermal transfer includes sublimation wherebymaterial dispersed in the transfer layer can be sublimated by heatgenerated in the donor element. A portion of the sublimated material cancondense on the receptor. The present invention contemplates transfermodes that include one or more of these and other mechanisms whereby theheat generated in an LTHC layer of a thermal mass transfer donor elementcan be used to cause the transfer of materials from a transfer layer toreceptor surface.

A variety of radiation-emitting sources can be used to heat thermal masstransfer donor elements. For analog techniques (e.g., exposure through amask), high-powered light sources (e.g., xenon flash lamps and lasers)are useful. For digital imaging techniques, infrared, visible, andultraviolet lasers are particularly useful. Suitable lasers include, forexample, high power (≧100 mW) single mode laser diodes, fiber-coupledlaser diodes, and diode-pumped solid state lasers (e.g., Nd:YAG andNd:YLF). Laser exposure dwell times can vary widely from, for example, afew hundredths of microseconds to tens of microseconds or more, andlaser fluences can be in the range from, for example, about 0.01 toabout 5 J/cm² or more. Other radiation sources and irradiationconditions can be suitable based on, among other things, the donorelement construction, the transfer layer material, the mode of thermalmass transfer, and other such factors.

When high spot placement accuracy is required (e.g., for highinformation full color display applications) over large substrate areas,a laser is particularly useful as the radiation source. Laser sourcesare also compatible with both large rigid substrates (e.g., 1 m×1 m×1.1mm glass) and continuous or sheeted film substrates (e.g., 100 μmpolyimide sheets).

During imaging, the thermal mass transfer element can be brought intointimate contact with a receptor (as might typically be the case forthermal melt-stick transfer mechanisms) or the thermal mass transferelement can be spaced some distance from the receptor (as can be thecase for ablative transfer mechanisms or transfer material sublimationmechanisms). In at least some instances, pressure or vacuum can be usedto hold the thermal transfer element in intimate contact with thereceptor. In some instances, a mask can be placed between the thermaltransfer element and the receptor. Such a mask can be removable or canremain on the receptor after transfer. A radiation source can then beused to heat the LTHC layer (and/or other layer(s) containing radiationabsorber) in an imagewise fashion (e.g., digitally or by analog exposurethrough a mask) to perform imagewise transfer and/or patterning of thetransfer layer from the thermal transfer element to the receptor.

Typically, selected portions of the transfer layer are transferred tothe receptor without transferring significant portions of the otherlayers of the thermal mass transfer element, such as the optionalinterlayer or the LTHC layer. The presence of the optional interlayermay eliminate or reduce the transfer of material from the LTHC layer tothe receptor and/or reduce distortion in the transferred portion of thetransfer layer. Preferably, under imaging conditions, the adhesion ofthe optional interlayer to the LTHC layer is greater than the adhesionof the interlayer to the transfer layer. In some instances, a reflectiveinterlayer can be used to attenuate the level of imaging radiationtransmitted through the interlayer and reduce any damage to thetransferred portion of the transfer layer that may result frominteraction of the transmitted radiation with the transfer layer and/orthe receptor. This is particularly beneficial in reducing thermal damagewhich may occur when the receptor is highly absorptive of the imagingradiation.

During laser exposure, it may be desirable to minimize formation ofinterference patterns due to multiple reflections from the imagedmaterial. This can be accomplished by various methods. The most commonmethod is to effectively roughen the surface of the thermal transferelement on the scale of the incident radiation as described in U.S. Pat.No. 5,089,372. This has the effect of disrupting the spatial coherenceof the incident radiation, thus minimizing self interference. Analternate method is to employ an antireflection coating within thethermal transfer element. The use of anti-reflection coatings is known,and may consist of quarter-wave thicknesses of a coating such asmagnesium fluoride, as described in U.S. Pat No. 5,171,650.

Large thermal transfer elements can be used, including thermal transferelements that have length and width dimensions of a meter or more. Inoperation, a laser can be rastered or otherwise moved across the largethermal transfer element, the laser being selectively operated toilluminate portions of the thermal transfer element according to adesired pattern. Alternatively, the laser may be stationary and thethermal transfer element and/or receptor substrate moved beneath thelaser.

In some instances, it may be necessary, desirable, and/or convenient tosequentially use two or more different thermal transfer elements to forma device, such as an optical display. For example, a black matrix may beformed, followed by the thermal transfer of a color filter in thewindows of the black matrix. As another example, a black matrix may beformed, followed by the thermal transfer of one or more layers of a thinfilm transistor. As another example, multiple layer devices can beformed by transferring separate layers or separate stacks of layers fromdifferent thermal transfer elements. Multilayer stacks can also betransferred as a single transfer unit from a single donor element.Examples of multilayer devices include transistors such as organic fieldeffect transistors (OFETs), organic electroluminescent pixels and/ordevices, including organic light emitting diodes (OLEDs). Multiple donorsheets can also be used to form separate components in the same layer onthe receptor. For example, three different color donors can be used toform color filters for a color electronic display. Also, separate donorsheets, each having multiple layer transfer layers, can be used topattern different multilayer devices (e.g., OLEDs that emit differentcolors, OLEDs and OFETs that connect to form addressable pixels, etc.).A variety of other combinations of two or more thermal transfer elementscan be used to form a device, each thermal transfer element forming oneor more portions of the device. It will be understood other portions ofthese devices, or other devices on the receptor, may be formed in wholeor in part by any suitable process including photolithographicprocesses, ink jet processes, and various other printing or mask-basedprocesses.

Referring back to the donor constructions shown in FIGS. 1(a)-(d),various layers of thermal mass transfer donor elements of the presentinvention will now be described.

Optional donor substrate 110 can be a polymer film. One suitable type ofpolymer film is a polyester film, for example, polyethyleneterephthalate or polyethylene naphthalate films. However, other filmswith sufficient optical properties, including high transmission of lightat a particular wavelength, as well as sufficient mechanical and thermalstability for the particular application, can be used. The donorsubstrate, in at least some instances, is flat so that uniform coatingscan be formed. The donor substrate is also typically selected frommaterials that remain stable despite heating of the LTHC layer. Thetypical thickness of the donor substrate ranges from 0.025 to 0.15 mm,preferably 0.05 to 0.1 mm, although thicker or thinner donor substratesmay be used.

The materials used to form the donor substrate and the adjacent layer(e.g., an underlayer or a LTHC layer) can be selected to improveadhesion between the donor substrate and the adjacent layer, to controltemperature transport between the substrate and the adjacent layer, tocontrol imaging radiation transport to the LTHC layer, and the like. Anoptional priming layer can be used to increase uniformity during thecoating of subsequent layers onto the substrate and also increase thebonding strength between the donor substrate and adjacent layers. Oneexample of a suitable substrate with primer layer is available fromTeijin Ltd. (Product No. HPE100, Osaka, Japan).

An optional underlayer 118 (as shown in FIGS. 1(c) and (d)) may becoated or otherwise disposed between a donor substrate and the LTHClayer, for example to minimize damage, such as thermal damage, to thedonor substrate during imaging. The underlayer can also influenceadhesion of the LTHC layer to the donor substrate element. Typically,the underlayer has high thermal resistance (i.e., has a lower thermalconductivity than the substrate) and acts as a thermal insulator toprotect the substrate from heat generated in the LTHC layer.Alternatively, an underlayer that has a higher thermal conductivity thanthe substrate can be used to enhance heat transport from the LTHC layerto the substrate, for example to reduce the occurrence of imagingdefects that can be caused by LTHC layer overheating.

Suitable underlayers include, for example, polymer films, metal layers(e.g., vapor deposited metal layers), inorganic layers (e.g., sol-geldeposited layers and vapor deposited layers of inorganic oxides (e.g.,silica, titania, aluminum oxide and other metal oxides)), andorganic/inorganic composite layers. Organic materials suitable asunderlayer materials include both thermoset and thermoplastic materials.Suitable thermoset materials include resins that may be crosslinked byheat, radiation, or chemical treatment including, but not limited to,crosslinked or crosslinkable polyacrylates, polymethacrylates,polyesters, epoxies, and polyurethanes. The thermoset materials may becoated onto the donor substrate or LTHC layer as, for example,thermoplastic precursors and subsequently crosslinked to form acrosslinked underlayer.

Suitable thermoplastic materials include, for example, polyacrylates,polymethacrylates, polystyrenes, polyurethanes, polysulfones,polyesters, and polyimides. These thermoplastic organic materials may beapplied via conventional coating techniques (for example, solventcoating, spray coating, or extrusion coating). Typically, the glasstransition temperature (Tg) of thermoplastic materials suitable for usein the underlayer is 25° C. or greater, preferably 50° C. or greater,more preferably 100° C. or greater, and, most preferably, 150° C. orgreater. In some embodiments, the underlayer includes a thermoplasticmaterial that has a Tg greater than any temperature attained in thetransfer layer during imaging. The underlayer may be eithertransmissive, absorbing, reflective, or some combination thereof, to oneor more wavelengths of imaging radiation.

Inorganic materials suitable as underlayer materials include, forexample, metals, metal oxides, metal sulfides, and inorganic carboncoatings, including those materials that are transmissive, absorptive,or reflective at the imaging light wavelength. These materials may becoated or otherwise applied via conventional techniques (e.g., vacuumsputtering, vacuum evaporation, or plasma jet deposition).

The underlayer may provide a number of benefits. For instance, theunderlayer may be used to manage or control heat transport between theLTHC layer and the donor substrate. An underlayer may be used toinsulate the substrate from heat generated in the LTHC layer or totransport heat away from the LTHC layer toward the substrate. It will beappreciated from the teaching of the present invention that temperaturemanagement and heat transport in the donor element can be accomplishedby adding layers and/or by controlling layer properties such as thermalconductivity (e.g., either or both the value and the directionality ofthermal conductivity), distribution and/or orientation of absorbermaterial, the morphology of layers or particles within layers (forexample, the orientation of crystal growth or grain formation inmetallic thin film layers or particles), and the like.

The underlayer may contain additives, including, for example,photoinitiators, surfactants, pigments, plasticizers, and coating aids.The thickness of the underlayer may depend on factors such as, forexample, the material of the underlayer, the material and opticalproperties of the LTHC layer, the material of the donor substrate, thewavelength of the imaging radiation, the duration of exposure of thethermal transfer element to imaging radiation, and the overall donorelement construction. For a polymeric underlayer, the thickness of theunderlayer typically is in the range of 0.05 μm to 10 μm, preferably,from about 0.1 μm to 4 μm, more preferably, 0.5 to 3 μm, and, mostpreferably, 0.8 to 2 μm. For inorganic underlayers (e.g., metal or metalcompound underlayer), the thickness of the underlayer typically is inthe range of 0.005 μm to 10 μm, preferably, from about 0.01 μm to 4 μm,and, more preferably, from about 0.02 to 2 μm.

Referring again to FIGS. 1(a)-(d), an LTHC layer 112 can be included inthermal mass transfer elements of the present invention to coupleirradiation energy into the thermal transfer element. The LTHC layerpreferably includes a radiation absorber that absorbs incident radiation(e.g., laser light) and converts at least a portion of the incidentradiation into heat to enable transfer of the transfer layer from thethermal transfer element to the receptor.

According to the present invention, LTHC layers can have anon-homogeneous distribution of absorber material, for example tocontrol a maximum temperature attained in the donor element and/or tocontrol a temperature attained at the transfer layer interface. Forexample, an LTHC layer can have absorber material distribution that isless dense closer to the donor substrate and more dense closer to thetransfer layer. In many instances, such a design can cause moreradiation to be absorbed and converted into heat deeper into the LTHClayer as compared to a homogeneous LTHC layer having the same thicknessand optical density. For the sake of clarity, the term “depth” when usedto describe a position in the LTHC layer means distance into the LTHClayer in the thickness dimension as measured from the donor substrateside of the thermal mass transfer element. In other instances, it may bebeneficial to have a LTHC layer having an absorber material distributionthat is more dense closer to the donor substrate and less dense closerto the transfer layer. Other examples of LTHC constructions arediscussed in more detail below.

Generally, the radiation absorber(s) in the LTHC layer absorb light inthe infrared, visible, and/or ultraviolet regions of the electromagneticspectrum and convert the absorbed radiation into heat. The radiationabsorber materials are typically highly absorptive of the selectedimaging radiation, providing an LTHC layer with an optical density atthe wavelength of the imaging radiation in the range of about 0.2 to 3or higher. Optical density is the absolute value of the logarithm (base10) of the ratio of a) the intensity of light transmitted through thelayer and b) the intensity of light incident on the layer.

Suitable radiation absorbing materials can include, for example, dyes(e.g., visible dyes, ultraviolet dyes, infrared dyes, fluorescent dyes,and radiation-polarizing dyes), pigments, metals, metal compounds, metalfilms, and other suitable absorbing materials. Examples of suitableradiation absorbers includes carbon black, metal oxides, and metalsulfides. One example of a suitable LTHC layer can include a pigment,such as carbon black, and a binder, such as an organic polymer. Anothersuitable LTHC layer includes metal or metal/metal oxide formed as a thinfilm, for example, black aluminum (i.e., a partially oxidized aluminumhaving a black visual appearance). Metallic and metal compound films maybe formed by techniques, such as, for example, sputtering andevaporative deposition. Particulate coatings may be formed using abinder and any suitable dry or wet coating techniques. LTHC layers canalso be formed by combining two or more LTHC layers containing similaror dissimilar materials. For example, an LTHC layer can be formed byvapor depositing a thin layer of black aluminum over a coating thatcontains carbon black disposed in a binder.

Dyes suitable for use as radiation absorbers in a LTHC layer may bepresent in particulate form, dissolved in a binder material, or at leastpartially dispersed in a binder material. When dispersed particulateradiation absorbers are used, the particle size can be, at least in someinstances, about 10 μm or less, and may be about 1 μm or less. Suitabledyes include those dyes that absorb in the IR region of the spectrum.For example, IR absorbers marketed by Glendale Protective Technologies,Inc., Lakeland, Fla., under the designation CYASORB IR-99, IR-126 andIR-165 may be used. A specific dye may be chosen based on factors suchas, solubility in, and compatibility with, a specific binder and/orcoating solvent, as well as the wavelength range of absorption.

Pigmentary materials may also be used in the LTHC layer as radiationabsorbers. Examples of suitable pigments include carbon black andgraphite, as well as phthalocyanines, nickel dithiolenes, and otherpigments described in U.S. Pat. Nos. 5,166,024 and 5,351,617,incorporated herein by reference. Additionally, black azo pigments basedon copper or chromium complexes of, for example, pyrazolone yellow,dianisidine red, and nickel azo yellow can be useful. Inorganic pigmentscan also be used, including, for example, oxides and sulfides of metalssuch as aluminum, bismuth, tin, indium, zinc, titanium, chromium,molybdenum, tungsten, cobalt, iridium, nickel, palladium, platinum,copper, silver, gold, zirconium, iron, lead, and tellurium. Metalborides, carbides, nitrides, carbonitrides, bronze-structured oxides,and oxides structurally related to the bronze family (e.g., WO_(2.9))may also be used.

Metal radiation absorbers may be used, either in the form of particles,as described for instance in U.S. Pat. No. 4,252,671, incorporatedherein by reference, or as films, as disclosed in U.S. Pat. No.5,256,506, incorporated herein by reference. Suitable metals include,for example, aluminum, bismuth, tin, indium, tellurium and zinc. Metalradiation absorbers that are magnetic may also be useful. Magneticparticles may be used as radiation absorbers in cases where a magneticfield might be used to orient magnetic particles, for example, or tonon-uniformly distribute magnetic particles in a binder that may behardened to fix the positions of the particles to form a non-homogeneousLTHC layer. For example, elongated or acicular magnetic particles can beused that have long dimensions that are smaller, but on the order of,the thickness of the LTHC layer, and that are oriented with their longdimension along the thickness direction of the LTHC layer. Otherorientations and distributions can be used. (e.g., 100 μm polyimidesheets).

Suitable binders for use in the LTHC layer include film-formingpolymers, such as, for example, phenolic resins (e.g., novolak andresole resins), polyvinyl butyral resins, polyvinyl acetates, polyvinylacetals, polyvinylidene chlorides, polyacrylates, cellulosic ethers andesters, nitrocelluloses, and polycarbonates. Suitable binders mayinclude monomers, oligomers, or polymers that have been, or can be,polymerized or crosslinked. Additives such as photoinitiators may alsobe included to facilitate crosslinking of the LTHC binder. In someembodiments, the binder is primarily formed using a coating ofcrosslinkable monomers and/or oligomers with optional polymer.

The inclusion of a thermoplastic resin (e.g., polymer) may improve, inat least some instances, the performance (e.g., transfer propertiesand/or coatability) of the LTHC layer. It is thought that athermoplastic resin may improve the adhesion of the LTHC layer to thedonor substrate. In one embodiment, the binder includes 25 to 50 wt. %(excluding the solvent when calculating weight percent) thermoplasticresin, and, preferably, 30 to 45 wt. % thermoplastic resin, althoughlower amounts of thermoplastic resin may be used (e.g., 1 to 15 wt. %).The thermoplastic resin is typically chosen to be compatible (i.e., forma one-phase combination) with the other materials of the binder. Asolubility parameter can be used to indicate compatibility, PolymerHandbook, J. Brandrup, ed., pp. VII 519-557 (1989), incorporated hereinby reference. In at least some embodiments, a thermoplastic resin thathas a solubility parameter in the range of 9 to 13 (cal/cm³)^(½),preferably, 9.5 to 12 (cal/cm³)^(½), is chosen for the binder. Examplesof suitable thermoplastic resins include polyacrylics, styrene-acrylicpolymers and resins, and polyvinyl butyral.

Conventional coating aids, such as surfactants and dispersing agents,may be added to facilitate the coating process. The LTHC layer may becoated onto the donor substrate using a variety of coating methods knownin the art. A polymeric or organic LTHC layer is coated, in at leastsome instances, to a thickness of 0.05 μm to 20 μm, preferably, 0.5 μmto 10 μm, and, more preferably, 1 μm to 7 μm. An inorganic LTHC layer iscoated, in at least some instances, to a thickness in the range of0.0005 to 10 μm, and preferably, 0.001 to 1 μm.

According to the present invention, thermal mass transfer donor elementscan include a non-homogeneous LTHC layer. For example, the LTHC layercan have a distribution of absorber material that varies with thickness.In particular, the LTHC layer can have an absorber density thatincreases with increasing depth. More generally, the LTHC layer can bedesigned to have a varying absorption coefficient by varying thedistribution or density of the same absorber material throughout theLTHC layer, or by including different absorber materials or layers indifferent locations in the LTHC layer, or both. For the purposes of thepresent disclosure, the term non-homogeneous includes anisotropicthermal properties or distributions of material(s) in at least onedirection in the LTHC layer.

Absorption coefficient is proportional to the rate of absorption ofimaging radiation in the LTHC layer. For a homogeneous LTHC layer, theabsorption coefficient is constant through the thickness, and theoptical density of the LTHC layer is approximately proportional to thetotal thickness of the LTHC layer multiplied by the absorptioncoefficient. For non-homogeneous LTHC layers, the absorption coefficientcan vary, complicating the calculation of an optical density. Exemplarynon-homogeneous LTHC layers have an absorption coefficient that variesas a function of thickness of the LTHC layer, and the optical densitywith depend on the integral of the absorption coefficient taken over theentire LTHC thickness range.

A non-homogeneous LTHC layer can also have an absorption coefficientthat varies in the plane of the layer. Additionally, absorber materialcan be oriented or non-uniformly dispersed in the plane of the LTHClayer to achieve an anisotropic thermal conductivity (e.g., acicularmagnetic particles can be used as absorber particles and can be orientedin the presence of a magnetic field). In this way, an LTHC layer can bemade that conducts thermal energy efficiently through the thickness ofthe layer to transport heat to the transfer layer while having poorthermal conductivity in the plane of the layer so that less heat isdissipated into adjacent, cooler areas, for example those areas thathave not been exposed to imaging radiation. Such an anisotropic thermalconductivity might be used to enhance the resolution of thermalpatterning using donor elements of the present invention.

Likewise, any of the other layers of a thermal mass transfer donorelement (e.g., substrate, underlayer, interlayer, and/or thermaltransfer layer) can be made to have anisotropic thermal conductivitiesto control heat transport to or away from other layers. One way to makelayers having anisotropic thermal conductivities is to have ananisotropic orientation or distribution of materials in the layer, thematerials having different thermal conductivities. Another way is imparta surface of one or more layers with a physical structure (e.g., to makea layer thinner in some spots and thicker in others).

By designing LTHC layers to have an absorption coefficient that varieswith layer thickness, imaging performance of the donor element can beenhanced. For example, the LTHC layer can be designed so that themaximum temperature attained in the donor element is lowered and/or thetransfer temperature (i.e., temperature attained at the transferlayer/LTHC interface or transfer layer/interlayer interface) is raised,relative to a homogeneous LTHC layer that has the same thickness andoptical density. Advantages can include the ability to use imagingconditions that can lead to improved transfer properties (e.g., transfersensitivity) without damaging the donor element or transferred patterndue to overheating of the donor.

In exemplary embodiments, thermal mass transfer donor elements of thepresent invention include an LTHC layer that has an absorptioncoefficient that varies with thickness. Such an LTHC layer can be madeby any suitable technique. For example, two or more layers can besequentially coated, laminated, extruded, or otherwise formed, each ofthe layers having a different absorption coefficient, thereby forming anoverall non-homogeneous LTHC layer. The boundaries between the layerscan be gradual (e.g., due to diffusion between the layers) or abrupt.Non-homogeneous LTHC layers can also be made by diffusing material intoa previously formed layer to create an absorption coefficient thatvaries with thickness. Examples include diffusing an absorber materialinto a binder, diffusing oxygen into a thin aluminum layer, and thelike.

Suitable methods for making non-homogeneous LTHC layers include, but arenot limited to: (i) sequentially coating two or more layers that haveabsorber material dispersed in a crosslinkable binder, each layer havinga different absorption coefficient, and either crosslinking after eachcoating step or crosslinking multiple layers together after coating allthe pertinent layers; (ii) sequentially vapor depositing two or morelayers that have different absorption coefficients; (iii) sequentiallyforming two or more layers that have different absorption coefficients,at least one of the layers including an absorber material disposed in acrosslinkable binder and at least one of the layers being vapordeposited, where the crosslinkable binder may be crosslinked immediatelyafter coating that particular layer or after other coating steps areperformed; (iv) sequentially extruding one or more layers, each layerhaving an absorber material disposed in a binder; (v) extruding amultiple layer stack of at least two layers, at least two of the layershaving absorber material dispersed therein to have different absorptioncoefficients; and (vi) any suitable combination or permutation of theabove. Examples of non-homogeneous LTHC layers that can be made includea two-layer structure that has a higher absorption coefficient in adeeper region, a two-layer structure that has a lower absorptioncoefficient in a deeper region, a three-layer structure that has anabsorption coefficient that becomes sequentially larger with depth, athree-layer structure that has an absorption coefficient that becomessequentially smaller with depth, a three-layer structure that has anabsorption coefficient that becomes larger and then smaller withincreasing depth, a three-layer structure that has an absorptioncoefficient that becomes smaller and then larger with increasing depth,and so on depending on the desired number of layers. With increasingnumbers of regions having different absorption coefficients, and/or withthinner regions, and/or with increased diffusion between regions, anon-homogeneous LTHC layer can be formed that approximates acontinuously varying absorption coefficient.

FIG. 2 compares power absorption and temperature rise for four donorelements having LTHC layers of the same thickness and overall opticaldensity (at the imaging wavelength) but different absorption coefficientprofiles. FIG. 2(a) shows the absorption coefficient profiles for thefour LTHC layers. One donor has a homogeneous LTHC profile 200, anotherdonor has a step-up (or dual layer) LTHC profile 202 such that theabsorption coefficient is constant at a lower level up to a certainthickness and then constant at a higher level thereafter, another donorhas a linearly increasing LTHC profile 204, and a final donor has agenerally exponentially increasing LTHC profile 206 that flattens out atthe deepest portion of the LTHC layer in order to give an opticaldensity that is the same as the other three donors, and to keep absorberloading levels within coatable limits.

FIG. 2(b) shows the power absorbed per unit volume as a function ofdepth for each of the four LTHC layer profiles 200, 202, 204, and 206,respectively, where the donor sheet is being irradiated from the shallowside of the LTHC layer. The power absorbed by the homogeneous LTHC layerdecreases continuously with depth into the LTHC layer. The powerabsorbed by the dual layer LTHC layer decreases continuously with depthuntil it reaches a sharp increase at the depth were the absorptioncoefficient changes, and then the power absorbed again decreases fromthis sharp increase. The power absorbed by the linear LTHC layerincreases to a maximum at some depth and then decreases through theremaining thickness. The power absorbed by the exponential LTHC layerremains constant throughout most of the LTHC layer until decreasing atthe depth where the absorption coefficient profile flattens out. As canbe observed from FIG. 2(b), the power absorption profiles for each ofthe non-homogeneous LTHC layers have a lower maximum than for thehomogeneous LTHC layer, as well as a higher minimum than for thehomogeneous LTHC layer. This result translates into the temperatureprofiles shown in FIG. 2(c).

FIG. 2(c) shows the relative temperature attained as a function of depthfor each of the four LTHC layer profiles 200, 202, 204, and 206,respectively as they are being irradiated from the shallow side of theLTHC layer. As can be seen, the maximum temperatures observed for eachof the non-homogeneous LTHC layers (indicated by 202, 204, and 206) arelower than the maximum temperature observed in the homogeneous LTHClayer (indicated by 200). Additionally, FIG. 2(c) gives informationabout the transfer temperature that can be achieved. The transfertemperature is governed, at least in part, by the heat generated at thedeepest portions of the LTHC layer. FIG. 2(c) demonstrates that the heatgenerated at the deepest portions for the non-homogeneous LTHC layers ishigher than that for the non-homogeneous LTHC layer. Thus, in general,non-homogeneous LTHC layers that have an absorption coefficient thatincreases with depth can be used to lower a maximum temperature attainedin the LTHC layer and to increase the donor element transfer temperaturewhen the donor element is irradiated from the shallow side of the LTHClayer.

An advantage to decreasing a maximum temperature in the donor elementcan be the reduction in defects caused by thermal decomposition oroverheating of the LTHC layer (or other layers). Such defects caninclude distortion of the transferred image (for example due todistortion or transparentization of the LTHC layer from excessive heatduring imaging), undesired transfer of portions of the LTHC layer to thereceptor, unintended fragmentation of the transferred image, increasedsurface roughness of the transferred image (for example due tomechanical distortion of one or more layers due to overheating of thedonor element during imaging), and the like. For convenience, suchdefects will be referred to collectively as imaging defects. Anotheradvantage to designing LTHC layers according to the present invention isthat higher power radiation sources and/or longer dwell times (e.g.,higher laser doses) can be used to raise the transfer temperature,thereby increasing the transfer fidelity, while still not exceeding atemperature in the LTHC layer that might lead to imaging defects.

Referring again to FIGS. 1(a) and (c), an optional interlayer 116 may bedisposed between the LTHC layer 112 and transfer layer 114, as shown fordonor constructions 100 and 104. The interlayer can be used, forexample, to minimize damage and contamination of the transferred portionof the transfer layer and may also reduce distortion in the transferredportion of the transfer layer. The interlayer may also influence theadhesion of the transfer layer to the rest of the thermal transfer donorelement. Typically, the interlayer has high thermal resistance.Preferably, the interlayer does not distort or chemically decomposeunder the imaging conditions, particularly to an extent that renders thetransferred image non-functional. The interlayer typically remains incontact with the LTHC layer during the transfer process and is notsubstantially transferred with the transfer layer.

Suitable interlayers include, for example, polymer films, metal layers(e.g., vapor deposited metal layers), inorganic layers (e.g., sol-geldeposited layers and vapor deposited layers of inorganic oxides (e.g.,silica, titania, and other metal oxides)), and organic/inorganiccomposite layers. Organic materials suitable as interlayer materialsinclude both thermoset and thermoplastic materials. Suitable thermosetmaterials include resins that may be crosslinked by heat, radiation, orchemical treatment including, but not limited to, crosslinked orcrosslinkable polyacrylates, polymethacrylates, polyesters, epoxies, andpolyurethanes. The thermoset materials may be coated onto the LTHC layeras, for example, thermoplastic precursors and subsequently crosslinkedto form a crosslinked interlayer.

Suitable thermoplastic materials include, for example, polyacrylates,polymethacrylates, polystyrenes, polyurethanes, polysulfones,polyesters, and polyimides. These thermoplastic organic materials may beapplied via conventional coating techniques (for example, solventcoating, spray coating, or extrusion coating). Typically, the glasstransition temperature (T_(g)) of thermoplastic materials suitable foruse in the interlayer is 25° C. or greater, preferably 50° C. orgreater, more preferably 100° C. or greater, and, most preferably, 150°C. or greater. The interlayer may be either transmissive, absorbing,reflective, or some combination thereof, at the imaging radiationwavelength.

Inorganic materials suitable as interlayer materials include, forexample, metals, metal oxides, metal sulfides, and inorganic carboncoatings, including those materials that are highly transmissive orreflective at the imaging light wavelength. These materials may beapplied to the light-to-heat-conversion layer via conventionaltechniques (e.g., vacuum sputtering, vacuum evaporation, or plasma jetdeposition).

The interlayer may provide a number of benefits. The interlayer may be abarrier against the transfer of material from the light-to-heatconversion layer. It may also modulate the temperature attained in thetransfer layer so that thermally unstable and/or temperature sensitivematerials can be transferred. For example, the interlayer can act as athermal diffuser to control the temperature at the interface between theinterlayer and the transfer layer relative to the temperature attainedin the LTHC layer. This may improve the quality (i.e., surfaceroughness, edge roughness, etc.) of the transferred layer. The presenceof an interlayer may also result in improved plastic memory or decreaseddistortion in the transferred material.

The interlayer may contain additives, including, for example,photoinitiators, surfactants, pigments, plasticizers, and coating aids.The thickness of the interlayer may depend on factors such as, forexample, the material of the interlayer, the properties of the LTHClayer, the properties of the transfer layer, the wavelength of theimaging radiation, and the duration of exposure of the thermal transferelement to imaging radiation. For polymer interlayers, the thickness ofthe interlayer typically is in the range of 0.05 μm to 10 μm. Forinorganic interlayers (e.g., metal or metal compound interlayers), thethickness of the interlayer typically is in the range of 0.005 μm to 10μm.

Referring again to FIGS. 1(a)-(d), a thermal transfer layer 114 isincluded in thermal mass transfer donor elements of the presentinvention. Transfer layer 114 can include any suitable material ormaterials, disposed in one or more layers with or without a binder, thatcan be selectively transferred as a unit or in portions by any suitabletransfer mechanism when the donor element is exposed to imagingradiation that can be absorbed by the LTHC layer and converted intoheat.

Examples of transfer layers that can be selectively patterned fromthermal mass transfer donor elements include transfer layers whichinclude colorants (e.g., pigments and/or dyes dispersed or dissolved ina binder), polarizers, liquid crystal materials, particles (e.g.,spacers for liquid crystal displays, magnetic particles, insulatingparticles, conductive particles), emissive materials (e.g., phosphorsand/or organic electroluminescent materials), hydrophobic materials(e.g., partition banks for ink jet receptors), hydrophilic materials,multilayer stacks (e.g., multilayer device constructions such as organicelectroluminescent devices), microstructured or nanostructured layers,photoresist, metals, polymer containing layers, adhesives, binders,enzymes or other bio-materials, or other suitable materials orcombination of materials. These and other transfer layers are disclosedin the following documents: U.S. Pat. Nos. 5,725,989; 5,710,097;5,693,446; 5,691,098; 5,685,939; and 5,521,035; InternationalPublication Nos. WO 97/15173, WO 98/03346, and WO 99/46961; andco-assigned U.S. patent application Ser. Nos. 09/231,724; 09/312,504;09/312,421; and 09/392,386.

Particularly well suited transfer layers include materials that areuseful in display applications. Thermal mass transfer according to thepresent invention can be performed to pattern one or more materials on areceptor with high precision and accuracy using fewer processing stepsthan for photolithography-based patterning techniques, and thus can beespecially useful in applications such as display manufacture. Forexample, transfer layers can be made so that, upon thermal transfer to areceptor, the transferred materials form color filters, black matrix,spacers, barriers, partitions, polarizers, retardation layers, waveplates, organic conductors or semi-conductors, inorganic conductors orsemi-conductors, organic electroluminescent layers, phosphor layers,organic electroluminescent devices, organic transistors, and other suchelements, devices, or portions thereof that can be useful in displays,alone or in combination with other elements that may or may not bepatterned in a like manner.

The receptor substrate may be any item suitable for a particularapplication including, but not limited to, glass, transparent films,reflective films, metals, semiconductors, various papers, and plastics.For example, receptor substrates may be any type of substrate or displayelement suitable for display applications. Receptor substrates suitablefor use in displays such as liquid crystal displays or emissive displaysinclude rigid or flexible substrates that are substantially transmissiveto visible light. Examples of rigid receptor substrates include glass,indium tin oxide coated glass, low temperature polysilicon (LTPS), andrigid plastic. Suitable flexible substrates include substantially clearand transmissive polymer films, reflective films, non-birefringentfilms, transfiective films, polarizing films, multilayer optical films,and the like. Suitable polymer substrates include polyester base (e.g.,polyethylene terephthalate, polyethylene naphthalate), polycarbonateresins, polyolefin resins, polyvinyl resins (e.g., polyvinyl chloride,polyvinylidene chloride, polyvinyl acetals, etc.), cellulose ester bases(e.g., cellulose triacetate, cellulose acetate), and other conventionalpolymeric films used as supports in various imaging arts. Transparentpolymeric film base of 2 to 100 mils (i.e., 0.05 to 2.54 mm) ispreferred.

For glass receptor substrates, a typical thickness is 0.2 to 2.0 mm. Itis often desirable to use glass substrates that are 1.0 mm thick orless, or even 0.7 mm thick or less. Thinner substrates result in thinnerand lighter weight displays. Certain processing, handling, andassembling conditions, however, may suggest that thicker substrates beused. For example, some assembly conditions may require compression ofthe display assembly to fix the positions of spacers disposed betweenthe substrates. The competing concerns of thin substrates for lighterdisplays and thick substrates for reliable handling and processing canbe balanced to achieve a preferred construction for particular displaydimensions.

If the receptor substrate is a polymeric film, it may be preferred thatthe film be non-birefringent to substantially prevent interference withthe operation of the display in which it is to be integrated, or it maybe preferred that the film be birefringent to achieve desired opticaleffects. Exemplary non-birefringent receptor substrates are polyestersthat are solvent cast. Typical examples of these are those derived frompolymers consisting or consisting essentially of repeating,interpolymerized units derived from 9,9-bis-(4-hydroxyphenyl)-fluoreneand isophthalic acid, terephthalic acid or mixtures thereof, the polymerbeing sufficiently low in oligomer (i.e., chemical species havingmolecular weights of about 8000 or less) content to allow formation of auniform film. This polymer has been disclosed as one component in athermal transfer receiving element in U.S. Pat. No. 5,318,938. Anotherclass of non-birefringent substrates are amorphous polyolefins (e.g.,those sold under the trade designation Zeonex™ from Nippon Zeon Co.,Ltd.). Exemplary birefringent polymeric receptors include multilayerpolarizers or mirrors such as those disclosed in U.S. Pat. Nos.5,882,774 and 5,828,488, and in International Publication No. WO95/17303.

EXAMPLES Example 1 Color Donor Elements

Two color donor elements were made, each having an overall constructionsuch as construction 100 shown in FIG. 1(a). The first color donorsheet, Color Donor 1, had a non-homogeneous LTHC layer, and the secondcolor donor sheet, Color Donor 2, had a homogeneous LTHC layer. ColorDonor 2 was used as a comparative example.

Color Donor 1: Non-homogeneous LTHC layer

Color Donor 1 was prepared in the following manner. A first LTHCsolution, given in Table I, was coated onto a 0.1 mm thick polyethyleneterapthalate (PET) film substrate. Coating was performed using a YasuiSeiki Lab Coater, Model CAG-150, using a microgravure roll with 150helical cells per lineal inch. The first LTHC coating was in-line driedat 80° C. and cured under ultraviolet (UV) radiation. The thickness ofthe cured coating was determined to be approximately 1.8 microns. Thecured coating had an optical density of 0.40 when measured usingincident light having a wavelength of 1064 nm. All optical densitiesreported in these examples were measured using the same wavelength ofincident light.

TABLE I First LTHC Coating Solution Parts by Component Trade DesignationWeight carbon black pigment Raven 760 Ultra⁽¹⁾ 1.99 polyvinyl butyralresin Butvar ™ B-98⁽²⁾ 0.35 acrylic resin Joncryl ™ 67⁽³⁾ 1.06dispersant Disperbyk ™ 161⁽⁴⁾ 0.60 surfactant FC-430⁽⁵⁾ 0.01 epoxynovolac acrylate Ebecryl 629⁽⁶⁾ 15.14 acrylic resin Elvacite 2669⁽⁷⁾10.10 2-benzyl-2-(dimethylamino)-1-(4- Irgacure ™ 369⁽⁸⁾ 1.02(morpholinyl) phenyl) butanone 1-hydroxycyclohexyl phenyl ketoneIrgacure ™ 184⁽⁸⁾ 0.15 2-butanone 43.55 1,2-propanediol monomethyl ether26.02 acetate ⁽¹⁾available from Columbian Chemicals Co., Atlanta, GA⁽²⁾available from Solutia Inc., St. Louis, MO ⁽³⁾available from S. C.Johnson & Son, Inc., Racine, WI ⁽⁴⁾available from Byk-Chemie USA,Wallingford, CT ⁽⁵⁾available from Minnesota Mining and ManufacturingCo., St. Paul, MN ⁽⁶⁾available from UCB Radcure Inc., N. Augusta, SC⁽⁷⁾available from ICI Acrylics Inc., Memphis, TN ⁽⁸⁾available fromCiba-Geigy Corp., Tarrytown, NY

Next, a second LTHC solution, as given in Table II, was coated onto thefirst cured LTHC coating. The second LTHC coating was coated in the samemanner as the first LTHC coating. The second LTHC coating was in-linedried at 80° C. and cured under ultraviolet (UV) radiation. Thethickness of the second cured coating was determined to be approximately0.9 microns. The second cured coating had an optical density of 0.79.The two LTHC coatings together formed a non-homogeneous LTHC layer onthe PET substrate. The non-homogeneous LTHC layer had an overallthickness of about 2.7 microns and an optical density of about 1.19.

TABLE II Second LTHC Coating Solution Parts by Component TradeDesignation Weight carbon black pigment Raven 760 Ultra 5.20 polyvinylbutyral resin Butvar ™ B-98 0.93 acrylic resin Joncryl ™ 67 2.78dispersant Disperbyk ™ 161 1.53 surfactant FC-430 0.02 epoxy novolacacrylate Ebecryl 629 6.08 acrylic resin Elvacite 2669 4.062-benzyl-2-(dimethylamino)-1-(4 Irgacure ™ 369 0.41 (morpholinyl)phenyl) butanone 1-hydroxycyclohexyl phenyl ketone Irgacure ™ 184 0.062-butanone 49.50 1,2-propanediol monomethyl ether acetate 29.43

An interlayer coating, given in Table III, was coated onto thenon-homogeneous LTHC layer by a rotogravure coating method using theYasui Seiki Lab Coater, Model CAG-150, with a microgravure roll having180 helical cells per lineal inch. This coating was in-line dried at 60°C. and UV cured. The thickness of the cured interlayer was determined tobe approximately 1.1 microns.

TABLE III Interlayer Coating Solution Parts by Component Weighttrimethylolpropane triacrylate esters 15.84 (SR 351 RP, available fromSartomer, Exton, PA) Butvar ™ B-98 0.99 Joncryl ™ 67 2.97 2-hydroxy-2methyl-1-phenyl-1-propanone 0.99 Duracure ™ 1173, available fromCiba-Geigy, Hawthorne, NY) 2-butanone 47.52 1-methoxy-2-propanol 31.68

A blue color transfer layer was formed onto the cured interlayer byrotogravure coating the solution given in Table IV using the Yasui SeikiLab Coater, Model CAG-150, with a microgravure roll having 180 helicalcells per lineal inch. This color transfer layer coating was in-linedried at 100° C. and was left un-cured. The thickness of the un-curedblue transfer layer was determined to be approximately 1.2 microns. Theaddition of the transfer layer completed Color Donor 1.

TABLE IV Blue Transfer Layer Coating Solution Parts by Component Weightblue pigment (Pigment Blue 15:6) 3.74 (Heliogen Blue L6700F, availablefrom BASF Corp., Mount Olive, NJ violet pigment (Pigment Violet 23) 0.16(HOSTAPERM Violet RL-NF, available from Clariant Corp., Coventry, RIDisperbyk ™ 161 1.59 dispersant 0.11 Solsperse 5000, available fromZeneca Inc., Wilmington, DE) Elvacite 2669 4.51 bisphenol A/novolacepoxy resin 4.00 (Epon SU-8, available from Shell chemical Co., Houston,TX) 1,2-propanediol monomethyl ether acetate 68.71 cyclohexanone 17.18

Color Donor 2 (Comparative): Homogeneous LTHC Layer

As a comparative example, Color Donor 2, was prepared having ahomogeneous LTHC layer. Color Donor 2 was prepared in the same manner asColor Donor 1, except that only one LTHC solution was coated on the PETsubstrate. The LTHC coating solution used is given in Table V. Thethickness of the resulting homogeneous LTHC layer was determined to beabout 2.8 microns, and it had an optical density of approximately 1.15.Thus, the overall thickness and optical density of the non-homogeneousLTHC of Color Donor 1 and the homogeneous LTHC of Color Donor 2 wereabout the same. An interlayer and color transfer layer were provided asabove to complete the construction of Color Donor 2.

TABLE V Homogeneous LTHC Coating Solution Parts by Component TradeDesignation Weight carbon black pigment Raven 760 Ultra 3.88 polyvinylbutyral resin Butvar ™ B-98 0.69 acrylic resin Joncryl ™ 67 2.07dispersant Disperbyk ™ 161 1.17 surfactant FC-430 0.01 epoxy novolacacrylate Ebecryl 629 13.18 acrylic resin Elvacite 2669 8.792-benzyl-2-(dimethylamino)-1-(4- Irgacure ™ 369 0.89 (morpholinyl)phenyl butanone 1-hydroxycyclohexyl phenyl ketone Irgacure ™ 184 0.132-butanone 43.37 1,2-propanediol monomethyl ether acetate 25.82

Example 2 Imaging of Color Donor Elements

Color Donor 1 and comparative Color Donor 2 were imaged from thesubstrate side of the donors using a laser imaging system to transfertheir respective transfer layers under various imaging conditions. Lasertransfer was accomplished using two single-mode Nd:YAG lasers. Scanningwas performed using a system of linear galvanometers, with the combinedlaser beams focused onto the image plane using an f-theta scan lens aspart of a near-telecentric configuration. The power on the image planewas approximately 16 W. The laser spot size, measured at the 1/e²intensity, was 30 microns by 350 microns. The linear laser spot velocitywas adjustable between 10 and 30 meters per second, measured at theimage plane. The laser spot was dithered perpendicular to the majordisplacement direction with about a 100 μm amplitude. The transferlayers were transferred as lines onto a glass receptor substrate, andthe intended width of the lines was about 90 μm. The glass receptorsubstrate was held in a recessed vacuum frame, the donor sheet wasplaced in contact with the receptor and was held in place viaapplication of a vacuum.

Color Donors 1 and 2 were imaged as a function of laser fluence, ordose, onto separate 1.1 mm thick glass receptors. The ramp-up distanceto full laser power was maintained at 500 microns for all doses. Thetransferred lines were then analyzed for width, edge roughness(calculated as the pooled standard deviation of the respective linewidth measurements), and the presence of certain imaging defects,specifically LTHC transfer to the receptor and/or fragmentation of thetransferred coating, collectively referred to in these Examples as“blow-up” defects. The results of these analyses are provided in tabularform in Table VI and in graphic form in FIG. 3. The bolded numbers inTable VI indicate results for the highest laser dose before reaching100% blow-up defects for each donor type.

TABLE VI Color Donor Imaging Performance as a Function of Laser DoseAverage Line Average Edge % of Lines with Dose Width (μm) Roughness (μm)Blow-up Defects (joules/cm²) Donor 1 Donor 2 Donor 1 Donor 2 Donor 1Donor 2 0.400 — 73 — 2.1 — 0 0.425 69 83 2.1 1.3 0 0 0.450 77 86 1.2 1.50 0 0.475 84 88 1.4 1.4 0 0 0.500 86 89 1.0 0.9 0 92 0.525 90 90 0.7 0.90 100 0.550 91 91 0.7 0.9 0 100 0.575 93 91 0.7 0.8 0 100 0.600 93 910.9 1.1 84 100 0.625 94 91 0.6 0.9 100 100 0.650 94 92 0.7 1.1 100 1000.675 95 91 0.7 0.8 100 100 0.700 96 91 0.7 0.9 100 100

FIG. 3(a) shows plots of average line width versus laser dose for ColorDonor 1 (given by line 300) and Color Donor 2 (given by line 310). Line302 indicates the laser dose at which Color Donor 1 started to showblow-up defects during transfer. Line 304 indicates the largest linewidth for lines transferred from Color Donor 1 before reaching theimaging dose indicated by line 302. Analogously, line 312 indicates thelaser dose at which Color Donor 2 started to show blow-up defects duringtransfer. Line 314 indicates the largest line width for linestransferred from Color Donor 2 before reaching the imaging doseindicated by line 312. The data shown in Table VI and the plots shown inFIG. 3(a) indicate that the non-homogeneous LTHC layer of Color Donor 1allows a higher laser dose to be used without forming blow-up defectsduring transfer. The data and plots also indicate that a higher averageline width was attainable using the non-homogeneous LTHC layer of ColorDonor 1, most probably due to higher laser doses that could be usedduring imaging of Color Donor 1 without blow-up.

A similar picture is evidenced in FIG. 3(b), which shows plots ofaverage edge roughness versus laser dose for Color Donor 1 (given byline 320) and Color Donor 2 (given by line 330). Lines 322 and 332indicate the laser doses at which Color Donor 1 and Color Donor 2,respectively, started to show blow-up defects during transfer. Lines 324and 334 indicate the smallest average edge roughness before reaching the“blow-up dose” for lines transferred using Color Donor 1 and Color Donor2, respectively. Using the non-homogeneous LTHC layer of Color Donor 1,a higher laser dose could be used without blow-up defects, which allowedlower edge roughnesses to be obtained.

The results of imaging of color donors indicates that donors using anon-homogeneous LTHC layer where the absorption coefficient is highernearer the transfer layer and lower nearer the direction of incidentimaging radiation can improve transfer (e.g., improve average line widthand edge roughness) than donors using a homogeneous LTHC layer andimaging in a similar manner. In addition, because of the lack of blow-updefects in Color Donor 1 within a range of laser doses that causedblow-up defects in Color Donor 2, the results indicate that a lowermaximum temperature was obtained for the same imaging conditions usingthe non-homogeneous LTHC layer of Color Donor 1 relative to thehomogeneous LTHC layer of Color Donor 2 that had nearly the samethickness and overall optical density.

Example 3 OLED Donor Elements

Two OLED donor elements were made, each having an overall constructionsuch as construction 100 shown in FIG. 1. The two OLED donor sheets,OLED Donor 1 and OLED Donor 2, were the same as to Color Donor 1 andColor Donor 2, respectively, as described above except in their transferlayers. The OLED donors had transfer layers formed onto their respectivecured interlayers by the following procedure.

Onto each OLED donor, a 100 Å thick layer of copper phthalocyanine wasdeposited on the interlayer as a release layer. A 450 Å thick layer ofaluminum was deposited on the release layer as a cathode layer. A 10 Åthick layer of lithium fluoride was then deposited on the aluminumcoating. A 500 Å thick layer of tris(8-hydroxyquinolinato)aluminum(ALQ₃) was deposited as an electron transport layer on the lithiumfluoride layer. Finally, a 500 Å thick layer ofN,N′-dinaphthyl-N,N′-diphenyl-4,4′-diamino biphenyl (NPB) was depositedas a hole transport layer on the electron transport layer. In this way,OLED donors were constructed having multicomponent transfer layers, thedonors designated OLED Donor 1, which had the non-homogeneous LTHC layerdescribed above, and OLED Donor 2, which had the homogeneous LTHC layerdescribed above.

All of the vacuum deposited materials were thermally evaporated anddeposited at room temperature. The deposition rate and thickness of eachvacuum deposited layer was monitored with a quartz crystal microbalance(Leybold Inficon Inc., East Syracuse, N.Y.). The background pressure(chamber pressure prior to the deposition) was roughly 1×10⁻⁵ Torr(1.3×10⁻³ Pa).

Example 4 Imaging of OLED Donor Elements

OLED Donor 1 and OLED Donor 2 were imaged as described above for thecolor donor elements as a function of dose onto 1.1 mm thick glassreceptors coated with indium tin oxide (“ITO”). The ramp-up distance tofull laser power was maintained at 500 microns for all doses. Thetransferred lines were then analyzed for the presence of LTHC blow-updefects (such as those described in Example 2). The results of theseanalyses are provided in tabular form in Table VII. During transfer, thehole transport layer of the OLED multicomponent transfer layer was incontact with the receptor substrate. When transferred, the order of thelayers in the transferred image were the same as on the donor elementsexcept that the outer most layer was the cathode layer and the innermost layer (contacting the receptor) was the hole transport layer.

TABLE VII OLED Donor Performance as a Function of Imaging Dose % ofLines with LTHC Imaging Dose Blow-up Defects (Joules/cm²) OLED Donor 1OLED Donor 2 0.400 partial transfer no transfer 0.450 0 0 0.500 0 00.550 0 40 0.600 0 80 0.650 0 100 0.700 0 100

The results indicate that higher imaging doses could be used withoutcausing blow-up defects for OLED Donor 1, which had a non-homogeneousLTHC layer, than for OLED Donor 2, which had a homogeneous LTHC layer.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the instant specification.

What is claimed is:
 1. A method for making a thermal mass transfer donorelement comprising the steps of: coating at least two layerssimultaneously by co-extrusion, each layer comprising an absorbermaterial dispersed in a crosslinkable binder, and crosslinking thecrosslinkable binders of the layers to form a non-homogeneouslight-to-heat conversion layer between a donor substrate and a thermaltransfer layer, wherein the donor element is capable of being used forimagewise thermal mass transfer of material from the thermal transferlayer to a proximately located receptor when the donor element isexposed to imaging radiation that can be absorbed and converted intoheat by the non-homogeneous light-to-heat conversion layer.
 2. Themethod of claim 1, wherein all of the at least two layers are coatedbefore the crosslinking step is performed.
 3. The method of claim 1,wherein crosslinking the crosslinkable binders of all the at least twolayers is performed simultaneously.
 4. A method for making a thermalmass transfer donor element comprising the steps of: coating at leasttwo layers, each layer comprising an absorber material dispersed in acrosslinkable binder, and crosslinking the crosslinkable binders of thelayers to form a non-homogeneous light-to-heat conversion layer betweena donor substrate and a thermal transfer layer, wherein all of the atleast two layers are coated before the crosslinking step is performed,wherein the donor element is capable of being used for imagewise thermalmass transfer of material from the thermal transfer layer to aproximately located receptor when the donor element is exposed toimaging radiation that can be absorbed and converted into heat by thenon-homogeneous light-to-heat conversion layer.
 5. The method of claim3, wherein the at least two layers are coated sequentially.
 6. Themethod of claim 4, wherein crosslinking the crosslinkable binders of allthe at least two layers is performed simultaneously.
 7. A method formaking a thermal mass transfer donor element comprising the steps of:coating at least two layers, each layer comprising an absorber materialdispersed in a crosslinkable binder, and crosslinking the crosslinkablebinders of the layers to form a non-homogeneous light-to-heat conversionlayer between a donor substrate and a thermal transfer layer, whereincrosslinking the crosslinkable binders of all the at least two layers isperformed simultaneously, wherein the donor element is capable of beingused for imagewise thermal mass transfer of material from the thermaltransfer layer to a proximately located receptor when the donor elementis exposed to imaging radiation that can be absorbed and converted intoheat by the non-homogeneous light-to-heat conversion layer.
 8. Themethod of claim 7, wherein the at least two layers are coatedsequentially.